Fixed bug introduced in googleapis#231

Author: Alejandro Casanovas

O365/message.py

@@ -542,7 +542,7 @@ def to_api_data(self, restrict_keys=None):
         if self.sender and self.sender.address:
             message[cc('from')] = self._recipient_to_cloud(self.sender)
 
-        if self.categories or 'categories' in restrict_keys:
+        if self.categories or 'categories' in (restrict_keys or {}):
             message[cc('categories')] = self.categories
 
         if self.object_id and not self.__is_draft: